The invention relates generally to operation of windfarms within an electric grid and more specifically to voltage control for a system of multiple windfarms.
Typically, an electric power system includes a plurality of power generation assets, which are spread over a geographic area. The electric power system also includes systems that consume power (loads) that may also be spread over the geographic area. The electric power system also includes a grid, a network of electric power lines and associated equipment used to transmit and distribute electricity over a geographic area. The infrastructure of the grid, may include, but is not limited to devices for interconnection, control, maintenance, and improvement of the electric power system operation. Typically, the electric power system includes a centralized control system operatively connected to the power generation assets for controlling a power output of each of the power generation assets, for example, using processing logic. The network operator usually operates the centralized control system. The power output of the power generation assets controlled by the centralized control system may include, but is not limited to an amount of electrical power, and a voltage for the electrical power.
Wind energy is often used to generate electrical power at power plants, often referred to as windfarms, using, for example, the rotation of large wind turbines to drive electrical generators. Windfarms and their associated windfarm controllers can control reactive power supply, and to a more limited extent active power. Larsen, in U.S. Pat. Nos. 7,119,452, 7,166,928, and U.S. Pat. No. 7,224,081 (assigned to General Electric Co.) describes a voltage control for wind generators including a farm-level controller with a reactive power command and a wind turbine generator control system. Wind turbine generator voltage control may be provided by regulating the voltage according to a reference set by a higher-than-generator-level (substation or farm level) controller. Reactive power may be regulated over a longer term (e.g. few seconds) while wind turbine generator terminal voltage is regulated over a shorter term (e.g. fraction of a second) to mitigate the effect of fast grid transients.
For economic reasons and as one of the approaches to reduce the environmental impacts of fossil fuel power generation, wind turbine generators with larger power output are being produced and windfarms with greater numbers of wind turbine generators are being brought into operation. The power output from the windfarms may comprise a significantly larger part of the total power being supplied and transmitted along the transmission grid. Often, an original windfarm may be sited at a certain geographic location, based on desirable wind conditions at that location. Later, one or more additional windfarms may be sited at the same geographic area, based on the desirable wind conditions that motivated the first windfarm. The later windfarms may be built by the same operator as the first windfarm or by completely different operators. The outputs from windfarms may be interconnected in a variety of points, which are ultimately tied together at a point of common coupling. The point of common coupling may also be the point of interconnection to the electric power system grid. The point of common coupling may provide a location for measurement of combined output parameters from the plurality of interconnected windfarms. Alternatively, the point of common coupling may be removed from the point of interconnection with grid. Increasingly, as windfarms are being located in geographic areas with valuable wind characteristics, the windfarms are remote from existing transmission lines of a grid. Increasingly, transmission lines of up to hundreds of miles need to be constructed to tie newly-built windfarms into the existing grid.
The interconnection of the windfarms in the windfarm system may be in different configurations. The distances between the windfarms may vary. Further, the point of physical connection with the grid may be remote from any of the individual windfarms and the point of common coupling. In the case of the plurality of interconnected windfarms with individual local windfarm controllers, individual local power-related commands may be provided to the individual local windfarm controllers from the central control system. Typically, the power-related commands provided to the local windfarm controller may direct the local windfarm controller to provide a specific power-related output at the point of common coupling. However, the plurality of individual local windfarm controllers cannot control at the point of common coupling because the power-related parameters at that point are a combination of the outputs from all of the individual windfarms.
Prior art windfarm systems have incorporated regulation of the voltage output from multiple windfarms at a location of measurement or a point of common coupling, For example, Cardinal et al. (U.S. application Ser. No. 12/181,658 assigned to General Electric Co.) describes a master reactive control device for regulating voltage output from multiple windfarms at a point of common coupling or a point of interconnection with a grid. In other instances, the regulation of voltage output for multiple windfarms at a point of regulation distant from the location at which the parameters may be measured, for example at the point of interconnection with the grid. However, voltage regulation at a single point associated with the output from the multiple windfarms may lead to violation of voltage limits at other locations on the transmission line or within individual windfarms. FIG. 1 illustrates a voltage profile at various points on a transmission line between a point of common coupling (POCC) with a windfarm and a point of interconnection (POI) with a grid. Planning criteria may typically require that rated power from the windfarm be delivered at the POI for voltages in the range of 0.95 power unit (PU) to 1.05 PU. However, depending upon the fraction of rated power and compensation schemes employed, voltage will vary along the transmission line between the POCC and the POI with the grid. It would be desirable to be able to control output of windfarms to maintain points on the transmission line within voltage specification during transmission line operation. Voltage or other limits could similarly be violated at other points in the system of windfarms, such as at collector bus outputs for individual windfarms.
Further, prior art has incorporated methods for distributing reactive load and windfarm voltage optimization for reduction of collector system losses within individual windfarms such as in Cardinal et al. (U.S. application Ser. No. 12/039028 assigned to General Electric Co.). FIG. 2 illustrates a prior art system for minimizing losses within a single windfarm by distribution of reactive power commands to individual wind turbines utilizing a loss minimization algorithm. The windfarm collector system 200 shows three wind turbine generators 201, 202, and 203, however, the number of wind turbine generators may be broadly extended in practical application. The wind turbine generators 201, 202 and 203 provide outputs P1+jQ1 (207), P2+jQ2 (208) and P3+jQ3 (209). Each wind turbine generator 201, 202 and 203 is tied to a collector bus 205 through a wind turbine generator connection transformer 210, 211 and 212, respectively, where the transformer presents an impedance Z1, Z2 and Z3 to the collector system. The wind turbine generator collection transformers 210, 211 and 212 may be located at varying physical distances 215, 216 and 217 from the collection bus 205 presenting different line resistance and reactance to the system (Z4, Z5 and Z6). A common path for one or more wind turbine generator loads may also be presented to the collector system such as 218 (Z7) between the collection bus 205 and wind farm main transformer 224. While the impedances are shown for illustrative purposes as discrete elements, it is recognized that they may represent distributed line elements, representing varying distances of line.
The collector bus 205 is tied through a point of common connection to a transmission grid 225 through wind farm transformer 224. Sensing devices at the POCC 220 may provide measured voltage, current, power factor, real power and reactive power signals to a windfarm control system. A control system is provided for the windfarm. A reference command is provided to the windfarm control system for control of real and reactive power. However, only the reactive load reference command signal QREF 230 and reactive measured load signal QM (measured) 235 are provided to summer 240. The output from summer 240 is provided to control functions H(s) 250 for determining relative load distribution to the individual wind turbine generators. Control functions H(s) 250 incorporates a loss minimization algorithm whose technical effect is to minimize windfarm collector system loss by assignment of reactive loads Q1 251, Q2 252 and Q3 253 based on losses resulting from Z1, Z2 and Z3 wind turbine generator connection transformer losses, from Z4, Z5 and Z6 line losses and Z7 line losses. Further the windfarm control algorithm may be subject to various constraints, one of which may be a power factor of approximately 0.95 at the POCC. Such methods, however, have not addressed loss optimization for multiple windfarms including a transmission line between the windfarms and a point of interconnection (POI) with a grid.
Accordingly, there is a need to provide voltage controls at a point of regulation for multiple windfarms, which also provides for constraints on voltage or other system parameters at other locations on the system of windfarms. Further there is a need optimize losses in more complex windfarm systems, including systems of multiple windfarms feeding transmission lines.